Method of determining an aqueous bisphenol-a concentration

ABSTRACT

An electrochemical cell that includes a working electrode, which comprises of is made of gold, with gold-coated carbon nanotubes secured thereon via a conductive binder, wherein the electrochemical cell is utilized to detect the presence of bisphenol-A, or to determine a concentration of bisphenol-A in a solution. Various embodiments of the electrochemical cell, a method of producing the electrochemical cell, and a method of using the electrochemical cell for determining a concentration of bisphenol-A in a solution are also provided.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to an electrochemical cell that includes aworking electrode that includes gold, and gold-coated carbon nanotubessecured thereon via a conductive binder. The electrochemical cell isutilized to detect the presence of BPA, and also to determine a BPAconcentration of a solution.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

Bisphenol-A (2,2-bis(4-hydroxyphenyl)propane, BPA) is a toxic compoundwhich has been extensively utilized in the plastic industry as a monomerfor producing epoxy-resins and polycarbonate. BPA is an omnipresentorganic compound and it can be unintentionally released into theenvironment to pollute rivers and ground water resources. Furthermore,BPA can also transfer into food and drinking water from food containersthat include polycarbonates and/or epoxy resins, such as infant feedingbottles, tableware, storage containers, food cans, and the like, andtherefore, people are regularly exposed to trace amounts of BPA fromvarious sources of drinking water and food items. Research has shownthat BPA interferes with hormonal activities by disrupting growth,development, and reproduction of hormones. BPA has also been shown to bea source of cancer. As a result, a rapid and reliable measurement of BPAis important for health protection.

Various techniques have been investigated to measure the concentrationof BPA in solutions. Exemplary techniques include i) liquidchromatographs coupled with electrochemical cells, UV/Vis, and/orfluorescence detectors, ii) liquid chromatographs coupled with massspectrometry, gas chromatography, and iii) gas chromatographs coupledwith mass spectrometry. Generally these techniques need expensive andcomplicated instruments, time-consuming sample preparations, and skilledoperators. Other methods, such as fluorimetry, enzyme-linkedimmunosorbent assay, flow injection chemiluminescence, andelectrochemical methods have also been employed for BPA detection. Amongthese methods, electrochemical techniques are more popular, because ofeconomical approaches of instrumentation, high sensitivity, portability,and simplicity for operators. In order to enhance the electrochemicalresponse of BPA some chemically modified electrodes as well aselectrochemical cells have been developed. These electrodes, whendisposed in the electrochemical cells, revealed moderate sensitivities,with poor selectivity. Furthermore, most of these electrochemical cellswere used to detect the presence of BPA in environmental samples, andnot for food sample detection. Therefore, it is necessary to seek afacile, cheap, stable and highly selective method to determine BPAconcentrations in the food and pharmaceutical industries.

Functional carbon nanotubes show promise for applications in miniaturebiological electronic devices and carcinogenic metallic ionsdetermination due to their small dimensions, strength and remarkablephysical properties. These nanoparticles have been the subject ofnumerous investigations in chemical, physical, biological, and materialsciences, since their discovery by Iijima in 1991. Depending on theiratomic structure, CNTs behave electrically as a metal or as asemiconductor. Functionalized multi-walled carbon nanotubes (fMWCNTs)have also attracted great attention in the past few years. Among thevarious applications of carbon nanotubes, carbon nanotube (CNT)/metalnanoparticle hybrid-modified electrodes have been developed for uses asfuel-cell catalysts and biosensors. For example, electrocatalysts for adirect methanol fuel cell (DMFC) were prepared by electrodeposition ofplatinum nanoparticles (PtNPs) on MWCNT/Nafion and single walled carbonnanotube (SWCNT)/Nafion electrodes using Nafion as binder (Wu et al., J.Power Sources 2007, 174, 148-158). In addition, a Pt-CNT/glucosebiosensor was also developed by incorporation of glucose oxidase (GOx)on a Pt-CNT electrode using a Pt_(nano)/SiO₂ composite matrix as abinder (Yang et al., Biosensors and Bioelectronics 2006, 21, 1125-1131).However, there is still a need for a simple and low-cost method toprepare an effective electrode with high durability, reactivity andstability and is useful in, for example, electrochemical sensingsystems. In addition, carbon nanotubes, when used in conjunction withother elements, have been shown to effectively detect the presence oforganic molecules and proteins. The patent WO 2015,168435 relates to anelectrode that includes a buckypaper of functionalized carbon nanotubes(e.g. single-walled or multi-walled carbon nanotubes), and goldnanoparticles dispersed in the buckypaper for sensing biomarkers. Inaddition, the U.S. Pat. App. No. 2013/0161066 relates to a carbonnanotube-loaded electrode and a method of producing it, wherein thecarbon nanotubes are functionalized with metalnanoparticles-encapsulated dendrimers to sense organic molecules.

In view of the forgoing, one objective of the present invention is toprovide an electrochemical cell that includes a working electrode, whichcomprises of or is made of gold, with gold-coated carbon nanotubessecured thereon via a conductive binder. The electrochemical cell isutilized to detect the presence of BPA, and also to determine a BPAconcentration of a solution.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect the present disclosure relates to anelectrochemical cell, including i) a working electrode that includesgold, ii) gold-coated carbon nanotubes that include a) carboxylic acidfunctionalized carbon nanotubes, b) gold nanoparticles bound to thecarboxylic acid functionalized carbon nanotubes, iii) a conductivebinder that binds the gold-coated carbon nanotubes to the workingelectrode, iv) a counter electrode disposed adjacent to the workingelectrode having a gap therebetween.

In one embodiment, the working electrode consists of gold. In anotherembodiment, the working electrode does not include a material selectedfrom the group consisting of a glassy carbon, a glass, a quartz, a glasswafer, a silicon wafer, a melted silica, and a transparent polymer.

In one embodiment, the counter electrode comprises at least one metalselected from the group consisting of platinum, silver, copper, andzinc.

In one embodiment, the conductive binder comprises at least one compoundselected from the group consisting of an alkyl acetate, a polyetheracetate, a conductive epoxy, a polythiophene, apolythiophene-poly(styrenesulfonate) copolymer, a polyacetylene, apolyaniline, a polypyrrole, and derivatives thereof.

In one embodiment, the carbon nanotubes are multi-walled carbonnanotubes.

In one embodiment, the gold-coated carbon nanotubes are in the form of abuckypaper.

In one embodiment, the gold-coated carbon nanotubes have a specificsurface area in the range of 50-500 m²/g.

In one embodiment, a diameter of each of the gold-coated carbonnanotubes is within the range of 5-20 nm.

In one embodiment, an amount of the gold nanoparticles in thegold-coated carbon nanotubes is within the range of 0.5-2.5 vol %.

In one embodiment, the gold-coated carbon nanotubes form a layer on theworking electrode with a thickness in the range of 10 to 1000 μm.

In one embodiment, the gold-coated carbon nanotubes form a layer on theworking electrode having pores in the size range of 0.5-5 nm.

In one embodiment, the electrochemical cell is disposed on a microchip,wherein the working electrode is circular having a diameter in the rangeof 1-10 mm, and wherein the counter electrode is disposedcircumferentially around the working electrode having a gaptherebetween.

According to a second aspect the present disclosure relates to a methodof determining a BPA concentration in a BPA-containing solution with theelectrochemical cell. The method involves i) contacting theBPA-containing solution with the working electrode and the counterelectrode of the electrochemical cell, ii) applying a voltage to theworking electrode and the counter electrode to oxidize at least aportion of BPA in the BPA-containing solution to produce an electriccurrent within the electrochemical cell, iii) determining the BPAconcentration in the BPA-containing solution based on the electriccurrent.

In one embodiment, the BPA concentration in the BPA-containing solutionis within the range of 1.0 nM to 1.0 M.

In one embodiment, the BPA concentration in the BPA-containing solutionis determined in a time range of 5-20 seconds after the contacting.

In one embodiment, the BPA-containing solution comprises BPA and one ormore of C₁-C₅ alcohols, C₁-C₅ alkoxy phenols, amino phenols, arylhalides, and halide ions, and the method has a BPA selectivity of atleast 90%.

In one embodiment, the voltage is up to 2.0 V.

According to a third aspect the present disclosure relates to a methodof producing the electrochemical cell, involving i) binding thegold-coated carbon nanotubes onto the working electrode with theconductive binder, ii) disposing the counter electrode adjacent to theworking electrode having a gap therebetween.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1A is a top-view of a microchip of the present disclosure.

FIG. 1B is a magnified image of the microchip, wherein anelectrochemical cell is disposed at the center of the microchip.

FIG. 1C is an illustration of the electrochemical cell of the microchip,wherein a working electrode is disposed at the center and a counterelectrode is secured circumferentially around the working electrode.

FIG. 1D represents the working electrode and gold-coated carbonnanotubes adhered thereon via a conductive binder.

FIG. 1E illustrates a strip form of the microchip.

FIG. 1F represents components of the microchip.

FIG. 2A is a UV/Vis spectrum of the gold-coated carbon nanotubes, intransmittance mode.

FIG. 2B is a Raman spectrum of the gold-coated carbon nanotubes.

FIG. 2C is a FTIR spectrum of the gold-coated carbon nanotubes, intransmittance mode.

FIG. 3 is an X-ray diffraction spectrum of the gold-coated carbonnanotubes.

FIG. 4A is an FESEM micrograph of the gold-coated carbon nanotubes.

FIG. 4B is a magnified FESEM micrograph of the gold-coated carbonnanotubes.

FIG. 5A is an I-V response of the electrochemical cell with and withoutthe gold-coated carbon nanotubes.

FIG. 5B is an I-V response of the electrochemical cell having thegold-coated carbon nanotubes, in the presence (with chemical) and in theabsence (without chemical) of a BPA-containing solution.

FIG. 5C is an I-V response of the electrochemical cell having thegold-coated carbon nanotubes, in the presence of a BPA-containingsolution having a BPA concentration in the range of 1.0 nM to 1.0 M.

FIG. 6 is a calibration curve of the electrochemical cell.

FIG. 7A is an FESEM micrograph of the gold-coated carbon nanotubes.

FIG. 7B is a mechanism of oxidation of BPA on the working electrode ofthe electrochemical cell.

FIG. 8A is an I-V response of the electrochemical cell having thegold-coated carbon nanotubes, in the presence of a BPA-containingsolution having BPA and one or more organic molecules selected from thegroup consisting of C₁-C₅ alcohols, C₁-C₅ alkoxy phenols, amino phenols,aryl halides, and halide ions.

FIG. 8B represents a sensitivity of the electrochemical cell to BPA overother organic molecules present in the BPA-containing solution.

FIG. 8C is an I-V response of the electrochemical cell when exposed to aBPA-containing solution at different times.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views.

According to a first aspect the present disclosure relates to anelectrochemical cell, including a working electrode 104 and a counterelectrode 106 disposed adjacent to the working electrode having a gap105 therebetween.

The electrochemical cell 102 provided herein may be used as anindicative tool to detect and/or determine the concentration of at leastone organic molecule selected from the group consisting of bis-phenoliccompounds (e.g. bisphenol-A, bisphenol-F, bisphenol-S, bisphenol-C,bisphenol-E, bisphenol-P, bisphenol-Z, etc.), C₁-C₅ alcohols (e.g.methanol, ethanol, propanol, isobutanol, n-butanol, n-pentanol), C₁-C₅alkoxy phenols (e.g. methoxy phenol, ethoxy phenol, etc.), aminophenols, and aryl halides (e.g. dichlorobenzene). The electrochemicalcell 102 may also be used to detect and/or determine the concentrationof halide ions (e.g. fluoride, chloride, bromide, and iodide). In apreferred embodiment, the electrochemical cell 102 is used as anindicative tool to detect and/or determine the concentration ofbisphenol-A (i.e. 2,2-bis(4-hydroxyphenyl)propane, or BPA). Theelectrochemical cell 102 provided herein may also be used to detectenvironmental pollutants or toxins.

In one embodiment, the working electrode 104 comprises or is made ofgold. The working electrode 104, however, preferably does not include amaterial selected from the group consisting of a glassy carbon, asilicon wafer, a melted silica, a glass, a quartz, a glass wafer, and atransparent polymer. Exemplary transparent polymers include, but are notlimited to polystyrene (PS), polycarbonate (PC), poly methylmethacrylate (PMMA), styrene acrylonitrile (SAN), styrene methylmethacrylate (SMMA), polyethylene terephthalate glycol-modified (PET-G),methyl metacrylate butadiene styrene (MBS), and/or any combinationthereof.

The working electrode 104 may also be a gold alloy having at least 50 wt%, preferably at least 60 wt %, or preferably at least 70 wt %, orpreferably at least 80 wt %, or preferably at least 90 wt %, orpreferably at least 95 wt % of gold, with the weight percent beingrelative to the total weight of the working electrode. In a preferredembodiment, the working electrode 104 is a gold alloy including gold andtitanium, wherein at least 70 wt %, or preferably at least 80 wt %, orpreferably at least 90 wt % of the working electrode is gold. In someembodiments, the working electrode 104 may include less than 20 wt %,preferably less than 15 wt %, more preferably less than 10 wt %, evenmore preferably less than 5 wt % of metal nanoparticles. The metalnanoparticles may be made of metals selected from the group consistingof Au, Ti, Ag, Cu, Pt, Pd, Ru, Re, Fe, and Ni. The term “metalnanoparticles” as used herein, generally refers to particles having anaverage diameter of less than 100 nm, preferably 10-50 nm, morepreferably 20-40 nm, even more preferably about 25 nm. In someembodiments, the metal nanoparticles may be bimetallic composites. Thebimetallic composites may include, but are not limited to, Pt—Ru, Pt—Ni,or a combination thereof. The bimetallic composites may havesubstantially similar shapes and/or substantially similar sizes.Alternatively, the bimetallic composites may have substantiallydifferent shapes and/or substantially different sizes. An electricalconductivity of the working electrode 104 may be within the range of3.0×10⁵-7.0×10⁵ s/cm, preferably 3.0×10⁵-6.5×10⁵ s/cm, more preferably5.0×10⁵-6.5×10⁵ s/cm.

In one embodiment, the counter electrode 106 comprises at least onemetal selected from the group consisting of platinum, silver, copper,palladium, indium, and zinc. In a preferred embodiment, the counterelectrode 106 is made of platinum or a platinum alloy. In the case ofthe platinum alloy, the counter electrode includes at least 50 wt %,preferably at least 60 wt %, or preferably at least 70 wt %, orpreferably at least 80 wt %, or preferably at least 90 wt %, orpreferably at least 95 wt % of platinum, with the weight percent beingrelative to the total weight of the counter electrode. The counterelectrode 106 may also include less than 20 wt %, preferably less than15 wt %, more preferably less than 10 wt %, even more preferably lessthan 5 wt % of metal nanoparticles, as described previously. In oneembodiment, an electrical conductivity of the counter electrode may bewithin the range of 3.0×10⁵-7.0×10⁵ s/cm, preferably 3.0×10⁵-6.5×10⁵s/cm, more preferably 5.0×10⁵-6.5×10⁵ s/cm.

The working electrode and the counter electrode are disposed adjacent toone another such that the gap 105 between the working electrode and thecounter electrode is less than 5 mm, preferably less than 2 mm, morepreferably less than 0.5 mm.

The electrodes provided herein may provide an electric signal byoxidizing or reducing an organic molecule selected from the groupconsisting of bis-phenolic compounds (e.g. bisphenol-A, bisphenol-F,bisphenol-S, bisphenol-C, bisphenol-E, bisphenol-P, bisphenol-Z, etc.),C₁-C₅ alcohols (e.g. methanol, ethanol, propanol, isobutanol, n-butanol,n-pentanol), C₁-C₅ alkoxy phenols (e.g. methoxy phenol, ethoxy phenol,etc.), amino phenols, and aryl halides (e.g. dichlorobenzene). Inaddition, the electrodes provided herein may be solid and free-standingfilms or rods having a structure that are easier to handle than theelectrode films formed by evaporating a dispersion from a surface ofglassy carbon.

The electrochemical cell 102 further includes gold-coated carbonnanotubes 110 deposited on a portion of an external surface of theworking electrode 104. For example, in one embodiment, at least 70%,preferably at least 80%, more preferably at least 90%, even morepreferably at least 95% of the external surface of the working electrode104 is coated with the gold-coated carbon nanotubes 110. One purpose ofhaving gold-coated carbon nanotubes in the electrochemical cell is toincrease an electrical conductivity as well as specific surface area ofthe working electrode. The gold-coated carbon nanotubes also may providea prolonged stability to the working electrode.

In a preferred embodiment, the gold-coated carbon nanotubes includemulti-walled carbon nanotubes (MWCNT) having a diameter within the rangeof 5-20 nm, preferably 8-15 nm, more preferably about 10 nm, and anaspect ratio of greater than or equal to about 5, preferably greaterthan or equal to about 100, more preferably greater than or equal toabout 1000. The multi-walled carbon nanotubes may be closed structureshaving hemispherical caps at each end of respective tubes, or they mayhave a single open end or both open ends. The multi-walled carbonnanotubes also may include a central hollow portion, which may be filledwith amorphous carbonaceous compounds.

In another embodiment, the gold-coated carbon nanotubes includesingle-walled carbon nanotubes (SWCNT) having a diameter within therange of 0.5-3 nm, preferably 1-2 nm, more preferably about 1.5 nm, andan aspect ratio of greater than or equal to about 50, preferably greaterthan or equal to about 100, more preferably greater than or equal toabout 1000. The single-walled carbon nanotubes may be closed structureshaving hemispherical caps at each end of respective tubes, or they mayhave a single open end or both open ends. The single-walled carbonnanotubes also may include a central hollow portion, which may be filledwith amorphous carbonaceous compounds.

The single-walled and multi-walled carbon nanotubes may be synthesizedby any method known in the art such as an arc discharge method, a laserablation method and a chemical vapor deposition (CVD) method.

Other carbonaceous compounds may also be used in addition to or as areplacement for the single-walled or multi-walled carbon nanotubes ofthe gold-coated carbon nanotubes. Exemplary carbonaceous compoundsinclude, but are not limited to carbon nanofibers, carbon nanorods,carbon nanowhiskers, graphene sheets, fullerenes, and graphite flakes.

In a preferred embodiment, the gold-coated carbon nanotubes 110 are inthe form of a buckypaper. A buckypaper may refer to a film-shapedaggregate of carbon nanotubes. Using the gold-coated carbon nanotubes inthe form of a buckypaper may result in an enhanced electricalconductivity of the working electrode 104 due to an enhancedelectroactive surface area. The buckypaper may have a thickness in therange of 10-1000 μm, preferably 100-1000 μm, more preferably 100-500 μm.In one embodiment, the carbon nanotubes are randomly oriented in thebuckypaper. In another embodiment, the carbon nanotubes aresubstantially aligned in the buckypaper. The carbon nanotubes in thebuckypaper may be aligned by methods known in the art, includingexposing a magnetic force to a dispersive solution of carbon nanotubesprior to filtering the solution. In one embodiment, the gold-coatedcarbon nanotubes of the buckypaper are in the form of aggregates havinga size range of 0.5-1000 μm, preferably 10-100 μm. The buckypaper,however, preferably does not include an electro-catalytic molecule suchas an enzyme or an antibody, and therefore the electrochemical cell thathas the buckypaper cannot be used as a biomarker.

In an alternative embodiment, the gold-coated carbon nanotubes 110 maybe deposited on the working electrode 104 to form a layer with athickness in the range of 10-1000 μm, preferably 100-1000 μm, morepreferably 100-500 μm.

The gold-coated carbon nanotubes 110 include carboxylic acidfunctionalized carbon nanotubes, wherein gold nanoparticles are bound tothe carboxylic acid functionalized carbon nanotubes.

The method of fabricating the gold-coated carbon nanotubes has beendiscussed in detail in the third aspect of the present disclosure. Inone embodiment, the carboxylic acid functionalized carbon nanotubes areproduced by treating the carbon nanotubes in an acid solution. An acidtreatment may impart the carbon nanotubes with carboxyl substituents.The acid solution may be one selected from the group consisting ofsulfuric acid, nitric acid, or a combination thereof. For example, inone embodiment, carbon nanotubes are contacted with a 3:1 mixture ofsulfuric acid and nitric acid for a sufficient time to impart the carbonnanotubes with a desired amount of carboxyl substituents. The carbonnanotubes may then be filtered and washed with deionized water.

In one embodiment, the carboxylic acid functionalized carbon nanotubesare used as precursors for preparing gold-coated carbon nanotubes.Carboxyl functional groups present on the carbon nanotubes may providestrong physical interactions between the carbon nanotubes and goldnanoparticles. Accordingly, the presence of these interactions canreduce the instability of the electrochemical cell due to a release ofthe gold nanoparticles from the carbon nanotubes. The carboxylfunctional groups may be present on sidewalls and/or hemisphericalendcaps of the carbon nanotubes. Accordingly, the gold nanoparticles maypreferably be bound to the exterior of the carbon nanotubes. However, insome embodiments, the gold nanoparticles may be present inside thecarbon nanotubes.

In one embodiment, the gold-coated carbon nanotubes 110 have a specificsurface area in the range of 50-500 m²/g, preferably 100-300 m²/g, morepreferably about 250 m²/g. In another embodiment, the gold-coated carbonnanotubes 110 have pores in the size range of 0.5-5 nm, preferably 0.5-3nm, more preferably 1-2.5 nm.

In one embodiment, the gold nanoparticles have an average diameter ofless than 100 nm, preferably in the range of 20.0-40.0 nm, morepreferably about 25.0 nm. The gold nanoparticles may have substantiallysimilar shapes and/or substantially similar sizes. Alternatively, thegold nanoparticles may have substantially different shapes and/orsubstantially different sizes.

In one embodiment, an amount of the gold nanoparticles in thegold-coated carbon nanotubes 110 is within the range of 0.5-2.5 vol %,preferably 0.5-1.5 vol %, with the volume percent being relative to thetotal volume of the gold-coated carbon nanotubes.

The electrochemical cell 102 further includes a conductive binder 108that immobilizes the gold-coated carbon nanotubes 110 by binding themonto an external surface of the working electrode 104. Accordingly, theconductive binder 108 is sandwiched between the gold-coated carbonnanotubes 110 and the working electrode 104. Although the conductivebinder is disposed between the gold-coated carbon nanotubes and theworking electrode, the gold-coated carbon nanotubes may still contactthe working electrode.

In one embodiment, the conductive binder 108 is in the form of a thinlayer having a thickness of 0.5-5 mm, preferably 1-3 mm. In a preferredembodiment, the conductive binder does not require a heat treatment andit can easily be applied onto the external surface of the workingelectrode 104 to for a thin layer.

In one embodiment, the conductive binder 108 includes at least onecompound selected from the group consisting of an alkyl acetate, apolyether acetate, a conductive epoxy, a polythiophene, apolythiophene-poly(styrenesulfonate) copolymer, a polyaniline, apolyacetylene, a polypyrrole, and derivatives thereof. A singleconductive polymer may be used alone, or a combination of severalconductive polymers may be used as the conductive binder. Suitablepolythiophenes include polythiophenes having unsubstituted thiophenerings, or thiophenes rings that are substituted with one or more ofsubstituent alkyl groups, halogen atoms, alkoxy groups, cyano groups, orthe like. Poly(3-alkylthiophene), poly(3,4-dialkylthiophene), andpoly(3,4-alkenedioxythiophene), such as poly(3,4-ethylenedioxythiophene)(PEDOT), are exemplary polythiophenes. Thepolythiophene-poly(styrenesulfonate) copolymers may also be used as theconductive binder, wherein polythiophene block of the copolymer is oneof the compounds as described previously. Example ofpolythiophene-poly(styrenesulfonate) copolymers includepoly(3,4-alkenedioxythiophene)-poly(styrenesulfonate) copolymers such aspoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate). Similarly, thebinder may include polypyrrole with unsubstituted pyrrole rings, orpolypyrrole rings that are substituted with one or more of substituentalkyl groups, halogen atoms, alkoxy groups, cyano groups, or the like.Poly(3-alkylpyrrole), poly(3,4-dialkylpyrrole), andpoly(3,4-alkenedioxypyrrole), or the like are exemplary polypyrroles.Suitable polyanilines include polyanilines having unsubstituted anilinerings, or polyaniline rings that are substituted with one or more ofsubstituent alkyl groups, halogen atoms, alkoxy groups, cyano groups orthe like may be used. Poly(n-alkylaniline), poly(arylamine),poly(phenylenediamine), and poly(aminopyrene), or the like are exemplarypolyanilines.

In one embodiment, the conductive binder 108 is a composite comprising apolymer and metal particles. The metal particles may be metalnanoparticles (described previously). The metal particles may also be inthe form of flakes, powders, rods, etc. for example, as a result ofmachining, drilling, or sawing a metal. The metal particles may be atleast one metal selected from the group consisting of Au, Ag, Cu, Pt,Pd, Ru, Re, Fe, and Ni. The metal particles may have substantiallysimilar shapes and/or substantially similar sizes. Alternatively, themetal particles may have substantially different shapes and/orsubstantially different sizes. The polymer of the conductive binder 108may be at least one polymer selected from the group consisting of anepoxy (i.e. the conductive epoxy), a vinyl ester, a nitrocellulose, apolysulfide, a polybutadiene, a polybutadiene-b-acrylic acid, apolybutadiene-b-acrylonitrile, a carboxyl terminated polybutadiene, apolyurethane, a hydroxy terminated polybutadiene, and a polyglycidylnitrate. In an alternative embodiment, the conductive binder 108 is acomposite comprising a polymer and non-metal conductive particles,wherein the non-metal conductive particles may be at least one non-metalselected from the group consisting of carbon nanofibers, graphenesheets, carbon nanotubes, fullerenes, graphite flakes, ormetal-substituted polyhedral oligomeric silsesquioxane (POSS), andwherein the polymer is as described. In case, where metal particles areused, an amount of the metal particles in the conductive binder 108 maybe within the range of 0.5-5 vol %, preferably 1-3 vol %, morepreferably about 2 vol %, with the volume percent being relative to thetotal volume of the conductive binder. Alternatively, in case, wherenon-metal conductive particles are used, an amount of the non-metalconductive particles in the conductive binder may be within the range of0.5-3.5 vol %, preferably 0.5-1.5 vol %, more preferably about 1 vol %,with the volume percent being relative to the total volume of theconductive binder. In some alternative embodiments, if metal particlesare used, the amount of the metal particles in the conductive binder 108may be within the range of 5-50 wt %, preferably 10-30 wt %, morepreferably 10-20 wt %. However, if non-metal particles are used, theamount of the non-metal conductive particles in the conductive bindermay be within the range of 5-15 wt %, preferably 5-10 wt %, with theweight percent being relative to the total weight of the conductivebinder. In another embodiment, the conductive binder includes 1-10 wt %,preferably 2-5 wt %, more preferably about 3 wt % of nafion. Anelectrical conductivity of the conductive binder 108 may be within therange of 0.5-1000 s/cm, preferably 500-1000 s/cm.

Referring to FIG. 1F. In one embodiment, the electrochemical cell 102 issecured on a microchip 100, which includes a voltage input 114 inelectrical communication with the working electrode 104, and a voltageoutput 116 in electrical communication with the counter electrode 106.

A current/voltage sensing circuit 122 is provided for measuring anapplied voltage and a produced electric current within theelectrochemical cell 102. The current/voltage sensing circuit 122 mayexist as an individual, a stand-alone circuit, or may exist as asub-circuit. In addition, a central processing circuit 120 is inelectrical communication with the current/voltage sensing circuit 122.

In one embodiment, a temperature sensor circuit 126 in electricalcommunication with the central processing circuit 120 is provided forcollecting temperature data of a solution being contacted with theelectrochemical cell 102, generating a data signal based on thetemperature data collected and transmitting the same to the centralprocessing circuit 120. The temperature sensor circuit 126 includes oneor more temperature sensing elements 128 in electrical communicationtherewith. The sensing element(s) may be positioned on the microchip100, positioned proximal to the microchip 100, and/or may be positionedat a location that is remote from the microchip 100, so as to allow fortemperature measurements at multiple locations within and outside theelectrochemical cell. The sensing element(s) may be selected from thegroup consisting of resistance temperature detectors (RTD's),thermistors, thermocouples, diodes, and mixtures thereof. In a preferredembodiment, the sensing element is a platinum RTD. Sensing elements andsupporting circuitry are known in the art and are commerciallyavailable. In one embodiment, the temperature sensor circuit 126 iscapable of measuring temperatures within the range of ˜50° C. to 250°C., preferably −50° C. to 150° C.

In one embodiment, the microchip 100 further includes atransmitter/receiver circuit 130 in electrical communication with thecentral processing circuit 120 for wirelessly transmitting signals to aprocessing device such as a computer, or receiving signals from remotelylocated electronic devices such as a sensors or programmable logiccontrollers. In response to signals received by the current/voltagesensing circuit 122, temperature sensor circuit 126, and/or thetransmitter/receiver circuit 130 (if provided), the central processingcircuit 120 transmits an output I-V signal 132 to a monitor device fordisplaying an I-V output of the electrochemical cell. In one embodiment,the transmitter/receiver circuit 130 wirelessly transmits the output I-Vsignal 132 to a monitor device for displaying the I-V output of theelectrochemical cell.

In one embodiment, the electrochemical cell 102 is secured onto themicrochip 100, wherein at least a portion of the microchip 100, which isnot configured to be in a contact with a target solution, is coveredwith an encapsulating material 112 which is substantially chemicallyinert with respect to the solution being contacted with theelectrochemical cell. Suitable encapsulating materials 112 include, butare not limited to polytetrafluoroethylenes, epoxies, silicones,polyurethanes, polyimides, silicone-polyimides, parylenes,polycyclicolefins, silicon-carbons, and benzocyclobutenes.

In one embodiment, a switch 124 is provided in the microchip 100, whichin electrical communication with the central processing circuit 120.Accordingly, the central processing circuit 120 is preprogrammed toactuate the switch 124 in response to signals from the current/voltage,temperature, and/or transmitter/receiver circuits 130, respectively. Inone embodiment, the central processing circuit 120 is preprogrammed toactuate the switch 124 in response to receiving one or more of thefollowing signals: (1) a signal from the current/voltage sensingcircuit, indicating the applied voltage to the electrochemical cellfalls outside a predetermined range; and (2) a signal from thetemperature sensor circuit 126, indicating the temperature detected byone or more temperature sensing elements 128 falls outside apredetermined range. As it is understood by those with ordinary skill inthe art, the acceptable operational ranges (i.e. the predeterminedranges) for a given electrochemical cell is governed by factors such ascomponents used to construct the electrochemical cell, the architectureof the electrochemical cell, and the application for which theelectrochemical cell is employed.

In a preferred embodiment, the electrochemical cell 102 is secured on amicrochip 100, wherein the working electrode 104 is circular having adiameter in the range of 1-10 mm, preferably 1-5 mm, more preferablyabout 1.5-2 mm, and wherein the counter electrode 106 is disposedcircumferentially around the working electrode having a gap therebetween(as shown in FIGS. 1A, 1B, and 1C).

In one embodiment, the microchip 100 may be in the form of a strip,wherein the electrochemical cell 102 is disposed thereon (as shown inFIG. 1E).

In a preferred embodiment, the microchip 100 is employed for detectingbisphenol-A. Furthermore, the microchip may be utilized for detectingorganic pesticides such as organophosphate.

According to a second aspect the present disclosure relates to a methodof determining a BPA concentration in a BPA-containing solution with theelectrochemical cell 102. The method in accordance with the secondaspect involves contacting the BPA-containing solution with the workingelectrode 104 and the counter electrode 106 of the electrochemical cell102.

Contacting the BPA-containing solution with the working electrode andthe counter electrode of the electrochemical cell may involvesubmersing, or partially submersing the electrochemical cell within theBPA-containing solution. In the embodiments where the electrochemicalcell is disposed on a microchip, the microchip may be shaped and sizedaccording to the size of the BPA-containing solution. For example, themicrochip can be as small as the size of a body vein for in-vivo BPAmeasurement of blood in a human body. Accordingly, the microchip can beimplanted or embedded, or the microchip may be associated with hooks,barbs, or other features known in the art that permit the microchip tobe implanted or embedded. In addition, contacting may also involvedisposing the BPA-containing solution with an amount sufficient togenerate a desirable signal onto the electrochemical cell by any meansknown in the art such as spraying, etc.

The BPA-containing solution may refer to any solution that contains BPA.Exemplary BPA-containing solutions may include tap water, wastewater,bottled water, bottled sodas, liquid in canned foods, canned juices, andthe like. In a preferred embodiment, the BPA concentration in theBPA-containing solution is within the range of 1.0 nM to 1.0 M,preferably 1.0 nM to 1.0 mM, although the electrochemical cell maydetermine the BPA concentration in the BPA-containing solution if theBPA concentration falls outside of these preferable ranges. In anotherembodiment, the electrochemical cell determines the presence ofbisphenol-A in a solution. In one embodiment, the BPA-containingsolution comprises BPA and one or more of C₁-C₅ alcohols (e.g. methanol,ethanol, propanol, isobutanol, n-butanol, n-pentanol), C₁-C₅ alkoxyphenols (e.g. methoxy phenol, ethoxy phenol, etc.), amino phenols, andhalides (e.g. dichlorobenzene), halide ions (e.g. fluoride, chloride,bromide, and iodide), and the like. Accordingly, the electrochemicalcell 102 has a BPA selectivity of at least 80%, preferably at least 85%,more preferably at least 90%. The term “BPA selectivity” of theelectrochemical cell refers to a molar ratio (in percentile) of BPA overother organic molecules that are oxidized on the working electrode 104.For example, if the BPA selectivity of the electrochemical cell is 90%,that means 90 mol % of all molecules that are oxidized on the workingelectrode is BPA. The BPA selectivity of the electrochemical cell may berelated to an oxidation of BPA on a surface of the working electrode.Further, it may be related to the kinetics of oxidation. For example, ata specified voltage, BPA may be more readily adsorbed onto the workingelectrode than other organic molecules.

The method in accordance with the second aspect further involvesapplying a voltage to the working electrode 104 and the counterelectrode 106 of the electrochemical cell 102 to oxidize at least aportion of BPA in the BPA-containing solution to produce an electriccurrent within the electrochemical cell.

Applying the voltage may involve connecting a positive side of a powersource 118 to the voltage input 114 and a negative side of the powersource 118 to the voltage output 116. In a preferred embodiment, thepower source 118 provides a DC current and the voltage is in the rangeof 0.0 to 2.0 V, preferably 0.0 to 1.5 V.

The method in accordance with the second aspect further involvesdetermining the BPA concentration in the BPA-containing solution basedon the electric current.

As used herein, the term “determining” refers to a quantitativemeasurement that indicates the BPA concentration in the BPA-containingsolution, for example, via a calibration curve that relates the electriccurrent to a BPA concentration. The calibration curve may relate toseveral characteristics of the electrochemical cell, for example, theconductivity of the working electrode, the conductive binder, and thecounter electrode, as well as the amount of the gold nanoparticleswithin the gold-coated carbon nanotubes, the thickness of the layer ofthe gold-coated carbon nanotubes, the type of the carbon nanotubes (e.g.SWCNT or MWCNT), and the like. The term “determining” may also refer toa qualitative measurement to determine the presence of BPA within asolution.

In one embodiment, the BPA concentration in the BPA-containing solutionis determined in a time range of 5-20 seconds, preferably 5-15 seconds,more preferably about 10 seconds, after contacting the BPA-containingsolution with the working electrode and the counter electrode.

In one embodiment, the electrochemical cell 102 has a sensitivity withinthe range of 2.00 to 10.00 μA·cm⁻²·mM⁻¹, more preferably 5.70μA·cm⁻²·mM⁻¹, with respect to the BPA concentration. The sensitivity ofthe electrochemical cell refers to an indication of how much electriccurrent is produced when the BPA concentration changes from a lowerbound to an upper bound. For example, if the BPA concentration variesfrom 1.0 nM to 1.0 mM, an electric current of about 2.00 to 10.00 μA,more preferably about 5.70 μA may be generated per 1.0 cm² of theelectrochemical cell.

According to a third aspect the present disclosure relates to a methodof producing the electrochemical cell 102.

In one embodiment, the method in accordance with the third aspectinvolves fabricating the gold-coated carbon nanotubes 110. Accordingly,the carbon nanotubes are first treated in an acid solution (e.g.sulfuric acid and/or nitric acid) to form a dispersion solution, whereinthe carbon nanotubes are dispersed within the acid solution. The carbonnanotubes may be functionalized with carboxylic acid, wherein the weightpercent of functionalized carboxylic acid of each carbon nanotube is inthe range of 0.01-10 wt %, preferably 0.5-5 wt %, more preferably 1-5 wt%. The carbon nanotubes may also be dispersed with a solvent that doesnot substantially adversely impact one or more desirable features of thecarbon nanotubes and the working electrode. For example, the carbonnanotubes may be dispersed with dimethylformamide (DMF), deionizedwater, or a combination thereof. The carbon nanotubes may be dispersedvia stirring, sonication, agitation, or a combination thereof. In oneembodiment, the dispersion solution is subjected to a magnetic force toat least substantially align the carbon nanotubes prior to filtering thecarbon nanotubes.

Next, the dispersion solution is diluted in water (preferably adeionized water). In a preferred embodiment, a surfactant is also addedto the dispersion solution to increase a solubility of the carbonnanotubes. The surfactant may be an anion surfactant such as SDS (sodiumdodecyl sulfate), LDS (lithium dodecyl sulfate), SDBS (sodiumdodecylbenzenesulfonate), SDSA (sodium dodecylsulfonate), and the like,a cation surfactant such as DTAB (dodecyltrimethylammonium bromide),CTAB (cetyltrimethylammonium bromide), and the like, or a non-polarsurfactant such as PVP (Brij-series, Tween-series, Triton X-series),poly(vinylpyrrolidone), polyethylene oxide-polybutyleneoxide-polyethylene oxide triblock copolymer, polyethyleneoxide-polyphenylene oxide-polyethylene oxide triblock copolymer, and thelike.

After that, the dispersion solution may be sonicated until the colorturns into black, and then filtered by any means known in the art to getcarboxylic acid functionalized carbon nanotubes. Filtering thedispersion solution may be achieved with a polycarbonate membrane. Thefiltering may be assisted by pressure or vacuum. After filtering, thecarboxylic acid functionalized carbon nanotubes may be washed one ormore times with deionized water and air dried. Alternatively, carboxylicacid functionalized carbon nanotubes can be purchased and used fromcommercial sources such as Sigma Aldrich.

The carboxylic acid functionalized carbon nanotubes are immersed in agold nanoparticle solution and then sonicated, preferablyultra-sonicated to form the gold-coated carbon nanotubes 110. The goldnanoparticle solution includes a gold precursor and one or more of water(preferably distilled water), an alcohol, and a co-solvent. Accordingly,the co-solvent is one organic compound selected from the groupconsisting of n-methyl-2-pyrrolidone (NMP), dimethyl acetamide (DMAc),dimethylformamide (DMF), cyclohexanone, ethylalcohol, and chlorobenzene,whereas the alcohol is selected from the group consisting of ethanol,methanol, and ethylene glycol. Exemplary gold precursors include, butare not limited to HAuCl₄, NaAuCl₄, AuCl₃, NaAuBr₄, KAuCl₄, and hydratesor solvates thereof. In one embodiment, a platinum precursor may beutilized instead of the gold precursor. Exemplary platinum precursorsinclude, but are not limited to Na₂PtCl₆, Na₂PtCl₄, H₂PtCl₄, H₂PtCl₆,and hydrates or solvates thereof. In another embodiment, a metalprecursor may be used instead of the gold precursor. The metal precursormay refer to a precursor material that includes a metal having apotential of more than +0.5 V, an oxidation state greater than zero, andcapable of being reduced to form a metal atom (e.g. Ag⁺, Pd²⁺, Cu²⁺,Co²⁺, Cr²⁺, Cr³⁺, Ni²⁺). Additionally, a reducing agent known in the artmay also be used in the gold nanoparticle solution. Examples of thereducing agent includes but not limited to trisodium citrate, sodiumborohydride (NaBH₄), citric acid, and lithium aluminum hydride (LiAlH₄).Preferably, the pH of the gold nanoparticle solution (or alternativelythe Pt or metal nanoparticle solutions) is adjusted to a pH range from 2to 7, more preferably at about 4. Also, a concentration of the alcoholin the gold nanoparticle solution is in the range of 10-70 vol %,preferably 30-50 vol %, and a concentration of the gold precursor in thegold nanoparticle solution is in the range of 0.001 μM-100 mM,preferably 0.01 μM-50 mM.

The gold-coated carbon nanotubes are filtered by any means known in theart, washed one or more times with deionized water, and air dried.

The method in accordance with the third aspect further involves bindingthe gold-coated carbon nanotubes 110 onto the working electrode 104 withthe conductive binder 108 (as described). Accordingly, a thin layer ofthe conductive binder 108 with a thickness of 0.5-5 mm, preferably 1-3mm, more preferably about 1 mm is disposed onto the working electrode,and the gold-coated carbon nanotubes may then be sprinkled onto theconductive binder, followed by air drying the conductive binder. Theterm “sprinkling” may refer to a way of securing the gold-coated carbonnanotubes or the buckypaper on to the working electrode (e.g. using arobot), wherein the electrochemical cell is disposed onto a microchip.

Binding the gold-coated carbon nanotubes onto the working electrode maybe performed via a spraying method such as atomizing (or wet atomizing),or gas dynamic cold spraying. The gold-coated carbon nanotubes may alsobe bound to the working electrode via stamping the working electrodeincluding the conductive binder into the gold-coated carbon nanotubes.

In another embodiment, the gold-coated carbon nanotubes are in the formof the buckypaper (as described), and the buckypaper is secured onto theconductive binder.

In another embodiment, the gold nanoparticles are premixed with apolymeric binder to form a gold nanoparticle paste. The polymeric bindermay be at least one selected from the group consisting of an epoxy (i.e.the conductive epoxy), a vinyl ester, a nitrocellulose, a polysulfide, apolybutadiene, a polybutadiene-b-acrylic acid, apolybutadiene-b-acrylonitrile, a carboxyl terminated polybutadiene, apolyurethane, a hydroxy terminated polybutadiene, and a polyglycidylnitrate. The gold nanoparticle paste may further be adhered onto theworking electrode, without having the conductive binder and thegold-coated carbon nanotubes bound thereon, although it is preferred tobind the gold-coated carbon nanotubes on to the gold nanoparticle paste.

The method in accordance with the third aspect further involvesdisposing the counter electrode adjacent to the working electrode havinga gap therebetween. The gap may have a rounded shape or a straightshape, with a 1-10 mm, preferably a 1-5 mm, more preferably a 3-5 mmdistance apart.

In some embodiments, the counter electrode is disposed adjacent to theworking electrode in a microchip, which is fabricated viaphotolithography. Accordingly, a silicon wafer, which is preferablywashed with an alcoholic solvent (e.g. isopropyl alcohol), is annealedin nitrogen and aluminium is sputtered thereon, followed by aphotolithography process on the sputtered aluminium. Next, a siliconnitride layer is deposited via chemical vapor deposition, and then padsurfaces of the working and the counter electrode are etched via anetching solution (e.g. hydrochloric acid, hydrofluoric acid, etc.), orvia plasma etching. After that, platinum is sputtered on the siliconnitride layer, and is further patterned via photolithography to form thecounter electrode on the microchip. A similar process is performed on adeposited gold on the silicon nitride layer to form the workingelectrode on the microchip. In a preferred embodiment, a binding layer(preferably titanium), is disposed on the pad surfaces of each of theworking electrode and the counter electrode to increase a bondingstrength between each of the working electrode and the counter electrodeto the silicon nitride layer. The binding layer may be disposed viaphotolithography. Accordingly, the counter electrode includes thesilicon nitride layer, a titanium layer, and a platinum layer, whereasthe working electrode, which is preferably circular, includes thesilicon nitride layer, a titanium layer, and a gold layer, such that theworking electrode is not in an electrical contact with the counterelectrode. An encapsulating material (as described earlier) may beutilized on a periphery of the microchip to prevent a leakage of theBPA-containing solution (or any test solution).

The examples below are intended to further illustrate protocols for theelectrochemical cell and the method of using thereof for detecting BPA,and are not intended to limit the scope of the claims.

Example 1—Preparation of Gold Nanoparticles

Analytical grade of bisphenol-A, ethyl acetate, disodium phosphate(Na₂HPO₄), butyl carbitol acetate, monosodium phosphate (NaH₂PO₄), andother chemicals was used and purchased from Sigma-Aldrich. 0.1 Mphosphate buffer phase (PBP) at pH 7.0 is prepared by mixing ofequimolar concentration of 0.2 M Na₂HPO₄ and 0.2 M NaH₂PO₄ solution in100.0 mL deionized water at room conditions. Stock solution of BPA(1.0M) was made using ultra-pure water from the purchased BPA chemical.As received BPA is used to make various concentrations (0.1 nM to 1.0 M)in DI water and used as a target analyte. 10.0 ml of 0.1 M PBS is keptconstant during whole measurements. Brunauer-Emmett-Teller (BET)measurements were investigated on autosorb nitrogen gas sorption system(Quantachrome Instruments) using nitrogen as the adsorbate. Au-decoratedfMWCNT materials were degassed for 12.0 hour at 200.0° C. prior tomeasurement. The nitrogen sorption curve was taken as 60 ptsadsorption/60 pts desorption (Equal timeout: 240/240 ads/des), with theBET surface area calculated using a 3 point BET analysis. Fouriertransform infrared spectra were recorded for Au-decorated fMWCNT with aspectrophotometer in the mid-IR range, which was obtained from Bruker.Morphology of Au-decorated fMWCNT was evaluated by FESEM instrument(FESEM; JSM-7600F, Japan). Energy dispersive X-ray analysis (XEDS) wasinvestigated for Au-decorated fMWCNT using FESEM-coupled XEDS from JEOL,Japan. The X-ray photoelectron spectroscopy (XPS) measurements wereexecuted on a Thermo Scientific K-Alpha KA1066 spectrometer forAu-decorated fMWCNT. A monochromatic AlKα X-ray radiation source wasused as excitation sources, where beam-spot size was kept in 300.0 μm.The spectra were recorded in the fixed analyzer transmission mode, wherepass energy was kept at 200.0 eV. The scanning of the X-ray spectra wasperformed at pressures less 10⁻⁸ Torr. X-ray diffractometer equippedwith Cu-Kα₁ radiation (A=1.5406 nm) by a generator voltage (˜40.0 kV)and current (˜35.0 mA) applied for this measurement. The Au-decoratedfMWCNT was investigated with UV/visible spectroscopy [Lamda-950, PerkinElmer, and Germany].

Gold nanoparticles were prepared in solution method under the controlstirring at room conditions. Initially, 20.0 ml aqueous solutioncontaining 0.025 μM HAuCl₄ and 0.025 μM trisodium citrate was dissolvedin a conical flask containing deionized water and then put the mixtureonto control magnetic stirring at room condition for 5 min. Then,ice-cold 0.1 M freshly prepared NaBH₄ solution (0.6 ml) was slowly addedto the first mixture under control stirring. During addition, thesolution twirled light pink, indicating the gold nanoparticle formation.In UV/vis spectra, an absorption band was found at approximately 532 nmin UV-visible spectra which confirmed that the gold nanoparticles werehomogeneously formed into the aqueous system.

Example 2—Preparation and Purification of Au-Decorated fMWCNT

Carbon nanotubes are high quality and quite pure, although somenanoparticles still exist in the purchased material as a by-product.fMWCNTs are formed either as isolated units or as nanotubes arranged inbundles; no attempt was made to separate the different configurations.The purification of common carbon nanotubes is of great importance sincemost carbon nanotube applications require materials of high quality.Acid treatment is a common way for purification of carbon nanotubes andhas constituted the first step in many different purification schemes.Nitric acid treatment is usually employed to remove metal catalysts,together with some of the amorphous carbon, but it can also oxidizecarbon atoms at the ends. Sonicating carbon nanotubes in nitric acidopens the ends of the carbon nanotubes and thereby introduces carboxylicacid groups at the ends or at defect sites of carbon nanotubes. Thepreparation of MWCNTs functionalized with carboxylic acid groups wascarried out as follows; first, soak the MWCNTs in 5.0 M nitric acid,ultrasonically disperse them for 6.0 min. Second, dilute with a largeamount of water and add a little Triton X-100 surfactant to increasesolubility, sonicate it to be a black solution. Third, filter the blacksolution with 0.2 μm diameter film and collect the nanotubes. Repeat thesecond and the third steps. The evidence for the formation of functionalcarboxyl group (peak at 1712 cm⁻¹) on the fMWCNT is exhibited by FTIRspectroscopy. Then the air-dried filtrate functional MWCNT (0.5 wt %)again dispersed in 10.0 ml gold nanoparticles solution (0.1 mM) for 2hours under ultra-sonication. The Au-nanoparticles decorated fMWCNTmixture was filter first and dried for 3 hours in the air and theninvestigated for the total characterization. The final product was alsoused for the chemical sensor development using tiny microchips withconducting coating binders by reliable I-V method.

Example 3—Construction of Tiny μ-Chips Using Photolithography Method

Electrochemical microchips were fabricated by conventionalphotolithographic technique, where electrodes and passivation layerswere developed on silicon wafer followed by dicing and packaging.Nitrogen-doped silicon wafers were prepared and overflowed by extra-purewater. In this step, all contaminations on the surface and native SiO₂layer are removed perfectly. At first, the wet oxidation was employedand then dry oxidation was executed, where wafers annealed in thenitrogen environment. Aluminium was sputtered with aluminum-1% Sitarget. Then the photolithograph processes were applied. Resist coating,baking, exposure, and development were employed by Kanto chemicals, andthen it was rinsed thoroughly by ionic water. Aluminium was etched byetching solution and resistance layer was removed perfectly by plasmaetching instrument. Then silicon wafers were cleaned by acetone,methanol, and finally by plasma simultaneously. Silicon nitride (SiN)layer was deposited by chemical vapour deposition and then pad electrodesurfaces were etched by reactive ion etching. Finally residual resistlayer was removed by plasma etching. After photolithographic process,platinum was sputtered by SP150-HTS. Then it was patterned by lift-offmethod, in which wafers was immersed into the remover, and then washedwith isopropyl alcohol. Photolithographic process was againinvestigated, where titanium sputtered as a binding layer, and then goldwas evaporated by deposition method. Finally, gold layer was patternedby lift-off method. Parylene passivation layer was formed for theprotection of the microchip from water. Photolithographic process wasperformed again for pad protection. Then parylene-dimer was evaporatedby deposition apparatus. Photolithography process was done again forpatterning. Parylene layer was patterned by etching. Finally,un-necessary resists were removed by acetone and then wafer is cleanedby isopropyl alcohol (IPA). Resist was coated on a whole surface of thesilicon wafer for protection during dicing process was executed. Siliconwafer was diced into pieces by dicing apparatus and stored into thedesiccators, when not in use. Resist on microchip surface was removed byacetone and cleaned with IPA. The opposite side of the chip was roughedby a sandpaper sheet for better adhesion and electrical stability. Themicrochip is bonded with die and packaged by silver paste. It was driedin a drying oven. Pads on chip were connected to the package throughgold wire with bonding machine. Finally, silicon-based adhesive was puton the periphery of the chip to protect pads and gold wire from samplesolution. Adhesive was dried for 24 hours at room temperature. Thesemiconductor smart microchips were fabricated on silicon wafer.Aluminium was sputtered to fabricate as wiring and bonding pads.Pt—Ti—TiN was sputtered on thermal oxide of silicon and patterned byphotolithography to fabricate counter electrode (CE). Ti—TiN layers wereused for strong adhesion. Au—Ti were sputtered and lithographed, whichmade circular working electrode (WE) with a diameter of 1.68 mm in thecentre of the microchip.

After electrodes fabrication, the parylene layer was fabricated byevaporation method as a passivation layer. The wafer was diced to 5.0 mmsquare microchips. This microchip was bonded to a package by silverpaste. Aluminium pads were connected to the package by gold wire.Finally, adhesive (Araldite, Hantsman, Japan) was put on the peripheryof the chip, which prevents target solution from contacting pads.

Example 4—Optical Analysis

The optical absorption spectra of Au-decorated MWCNT are accomplished byUV-vis. spectrophotometer in the visible range (400.0-800.0 nm). Fromthe absorption spectrum, it has been found the maximum wavelength forAu-decorated MWCNT is about 316.5 nm, which is presented in FIG. 2A.Bang-gap energy (E_(bg)) is calculated on the basis of the maximumabsorption band of Au-decorated MWCNT and found to be 3.9241 eV,according to the following equation (Eq. 1):

$\begin{matrix}{E_{bg} = {\frac{1240}{\lambda}({eV})}} & ( {{Eq}.\mspace{14mu} 1} )\end{matrix}$

where E_(bg) is the band-gap energy and λ_(max) is the wavelength(˜316.5 nm) of the Au-decorated fMWCNT. No extra peaks associated withimpurities are observed in the spectrums, which proved that thefunctionalized control crystallinity of Au-decorated fMWCNT materials.

Example 5—Raman Spectroscopy

Raman spectroscopy is one of the most powerful tools forcharacterization of carbon nanotubes. All allotropic forms of carbon areactive in Raman spectroscopy, fullerenes, carbon nanotubes, amorphouscarbon, polycrystalline carbon, etc. The position, width, and relativeintensity of bands are modified according to the carbon forms. Thistechnique provides valuable information about the structure of carbonnanotubes. Briefly, there is strong evidence for a diameter-selectiveresonant Raman scattering process. The tangential mode (TM) in the range1400-1700 cm⁻¹ gives information on the electronic properties of thetubes, while the analysis of the so called D band at around 1361 cm⁻¹provides information as to the level of disordered carbon. The size ofthe D band relative to the TM band is a qualitative measure of theformation of undesirable forms of carbon. In this experiment, it is used788-nm (semiconductor Sapphire Laser) excitation for checkingAu-decorated fMWCNT. This is the most direct evidence of Au-decoratedfMWCNT, which is directly detected by Raman spectroscopy. The Ramanspectrum of the Au-decorated fMWCNT shown in FIG. 2B, where the G-lineat 1599 cm⁻¹ originates from the graphitic sheets and the peak at 1361cm⁻¹ is related to the defects (disorder mode consistent with sidewallfunctionalization) in Au-decorated fMWCNT. From this, we can alsoconclude that the physical structure of the Au-decorated fMWCNT was notchanged with the only exception of the opened ends.

Example 6—FTIR Spectroscopy

FTIR studies have been performed in the range 400 to 4000/cm⁻¹ for theidentification of the functional group attached on the surface of theAu-decorated fMWCNT. FTIR spectroscopy has been used extensively in thestructural determination of molecules. FIG. 2C shows a comparative FTIRdata for the refluxed samples. As observed in the prepared sample thereis a signal with small C—C stretch (1598 cm⁻¹). By acid treatment quitea number of new peaks appear. The bands due to the C═O stretch are veryprominently seen in the range 1712 cm⁻¹ for the carboxylated fMWCNT. Thesample refluxed in 3:1 H₂SO₄:HNO₃ acid for 6-7 hrs shows a distinct bandat 1710 cm⁻¹, which can be assigned to the acid carbonyl-stretching mode(FIG. 2C). Another band exhibit in this functionalized sample is at 3396cm⁻¹ that is characteristic of O—H stretches. The C—C vibrations occurdue to the internal defects, and the O—H vibration is associated withthe amorphous carbon because amorphous carbon easily forms a bond withatmospheric air. However, the intensity of this O—H peak is relativelylower and shows that a lesser amount of amorphous carbon formed duringgrowth. The peak at 1598 cm⁻¹ can be associated with the stretching ofthe carbon nanotube backbone. So, the evidence for the formation offunctional carboxyl group (peak at 1712 cm⁻¹) on the Au-decorated fMWCNTis investigated by FTIR spectroscopy.

Example 7—BET Analysis

Brunauer-Emmett-Teller (BET) theory aims to explain the physicaladsorption of gas molecules (especially nitrogen gas) on a mesoporousfunctionalized carbon nanotubes and serves as the basis for an analysismethod for the measurement of the specific surface area of preparedAu-decorated fMWCNT. The average pore diameter and specific surface area(BET: surface and pore volume] were measured for Au-decorated fMWCNTusing a Quantochrome NOVA 1000 (Boynton Beach, Fla., USA). In order toestimate if any change of the physical structure occurs of Au-decoratedfMWCNT, the pore size distribution and the specific surface area wereinvestigated using multipoint BET analysis. The pore size is in therange between 1.232 nm to 2.153 nm and the Au-decorated fMWCNT aspecific surface area of 222.450 m²/g. Therefore, the observed surfacearea results from a formation of nanopores accessible for thenitrogen-gas, resulting in an increase of the capacitance of theAu-decorated fMWCNT. The experimental data exhibited that varying thesynthetic conditions, extensively affected the pore size distributionand specific surface area of obtained Au-decorated fMWCNT.

Example 8—Binding-Energy Analysis

XPS measurements were measured for Au-decorated fMWCNT to investigatethe chemical states of carbon and oxygen. XPS was used to determine thechemical state of the Au-decorated fMWCNT and their depth. Thespin-orbit peaks of the C1s binding energy for the samples appeared ataround 285.1 eV, which is in good agreement with the reference data forcarbon. The C1s spectrum shows a distinguished peak at 532.7 eV. Thepeak at 532.7 eV is assigned to lattice oxygen, which indicated thepresence of oxygen (ie, O₂) in the Au-decorated fMWCNT. The Au 4fspectrum of the Au-decorated fMWCNT exhibits gold nanoparticles to be ina single metallic state with distinguished Au4f_(7/2) and Au4f_(5/2)peaks at 83.9 and 88.2 eV respectively. XPS compositional analysesevaluated the co-existence of the three single-atoms in Au-decoratedfMWCNT. Therefore, it is concluded that the functionalized carbonnanotubes composites have tube-phase contained two different elements.Also, this conclusion is reliable with the XRD data significantly inthis investigation.

Example 9—Structural and Morphological Analysis

As described in the recent works, the significant diffraction pattern ofAu-decorated fMWCNT is appeared at 2θ of 26.1° as shown in FIG. 3. The2θ peaks is corresponded to (002) reflection planes or also known asinterlayered spacing between adjacent graphite layers, respectively. The(002) reflection peaks was observed at the same 20 values infunctionalized MWCNTs diffractions. From XRD patterns of thefunctionalized MWCNTs samples, it is shown that the XRD patterns aresimilar to the pristine fMWCNTs. From XRD patterns, it can be concludethat functionalized MWCNTs is still had same cylinder-wall structure andinter-planner spacing after the functionalization process. Thus, thestructure of Au-decorated fMWCNT is protected even after undergone thetreatment as confirmed from XRD analysis previously. The diameter ofAu-decorated fMWCNT was also calculated and confirmed using Scherrerformula (Eq. 2):

D=0.9π/(β cos θ)  (Eq. 2)

where λ is the wavelength of x-ray radiation, β is the full width athalf maximum (FWHM) of the peaks at the diffracting angle θ. The averagediameter of Au-decorated fMWCNT is close to ˜11.2 nm.

High resolution FESEM images of Au-decorated fMWCNT are displayed inFIG. 4A and FIG. 4B. The FESEM images of gold-aggregated on functionalmaterials are exhibited in tube-shapes. The average diameter ofAu-decorated fMWCNT is calculated in the range of 9.0 to 20.0 nm, whichis close to ˜11.9 nm.

It is clearly displayed from FESEM that the Au-decorated fMWCNT isexhibited in regular white spotted gold onto tube-shape CNT withhigh-density materials. Thus, the treatment of fMWCNTs with strongoxidizing agents causes severe etching of the graphitic surface of thematerial, leading to tubes of shorter length with a large population ofdisordered sites. Analysis of FESEM images of gold-decoratedfunctionalized MWCNTs allowed reliable length measurements of only a fewnanotubes, and all of them were between 9.0 to 20.0 nm, similar topristine material previously reported. It is also confirmed that theAu-decorated functional nanotubes are composed in Au-white-spot ontotube-shape CNT of aggregated Au-decorated-fMWCNT.

Example 10—Elemental Analysis

The X-ray electron dispersive spectroscopy (XEDS) investigation ofAu-decorated fMWCNT indicates the presence of C, Au, and O compositionin the functional materials. It is clearly displayed that preparedAu-decorated fMWCNT controlled only carbon and oxygen elements. Thecomposition of C, Au, and O is 93.63%, 1.09%, and 5.46% respectively. Noother peak related with any impurity has been detected in the XEDS,which confirms that the Au-decorated fMWCNT products are composed onlywith C, Au, and O elements. Here, after functionalization the finalproduct contains oxygen element in the nanotubes which confirm to theformation of carboxylic group (—COOH) in the carbon nanotubes. It isalso confirmed from the existence of Au-element that the functionalizedcarbon nanotubes is decorated by adsorbed and aggregated Au-atom on thesurface or defect area of the nanotubes.

Example 11—Detection of Bisphenol-A by Au-Decorated fMWCNT/MicrochipAssembly

The potential application of Au-decorated fMWCNT assembled ontomicrochip as chemical sensors (especially BPA analyte) has beenmonitored for detecting hazardous chemicals, which are not environmentalsafe. Improvement of Au-decorated fMWCNT on microchip as chemicalsensors is in the initial stage and no other reports are available withAu-decorated fMWCNT-fabricated-chips. The Au-decorated fMWCNT sensorshave advantages such as stability in air, non-toxicity, chemicalinertness, electrochemical activity, simplicity to assemble orfabrication, and bio-safe characteristics. As in the case of toxic BPAsensors, the phenomenon of reason is that the current response in I-Vmethod of Au-decorated fMWCNT considerably changes when aqueous BPA areadsorbed.

The Au-decorated fMWCNT were applied for modification of chemicalsensor, where BPA was measured as target analyte. The magnifiedconstruction view of internal microchip center (sensing area) ispresented in the FIGS. 1A, 1B, and 1C, wherein platinum line (PtE) andgold-central-circle onto microchip is employed as CE (counter electrode)and WE (working electrode) electrodes (potential sources oftwo-electrodes assembly system onto the microchips) respectively. Inaddition, FIG. 1D shows the components of the working electrode. Thefabricated-surface of Au-decorated fMWCNTs/microchips sensor was madewith conducting binders on the microchip surface, which is presented inthe FIG. 1D. The fabricated microchip electrode was placed into the ovenat low temperature (60.0° C.) for two hours to make it dry, stable, anduniform the surface totally. I-V signals of chemical sensor areanticipated having Au-decorated fMWCNT thin film as a function ofcurrent versus potential for hazardous BPA. The real electricalresponses of target BPA are investigated by simple and reliable I-Vtechnique using Au-decorated fMWCNTs/microchips. The time holding ofelectrometer was set for 1.0 sec. A significant amplification in thecurrent response with applied potential is noticeably confirmed.

In FIG. 5A shows the current responses of un-coated (gray-dotted) andcoated (dark-dotted) microchip working electrodes with Au-decoratedfMWCNT in absence of target BPA. With Au-decorated fMWCNT fabricatingsurface, the current signal is slightly reduced compared to uncoatedAu-decorated fMWCNTs/microchip surfaces, which indicates the surface isslightly inhibited with Au-decorated fMWCNT during the measurement ofI-V curve. The current changes for the without target (dark-dotted) andwith target analyte (pink-dotted) injecting of towards target BPA(having a volume of ˜25.0 μL and a BPA concentration of ˜0.1 μM) ontowith Au-decorated fMWCNTs/microchips is showed in FIG. 5B. A significantcurrent enhancement is exhibited with the Au-decorated fMWCNT modifiedmicrochips compared with uncoated microchips due to the presence ofmesoporous carbon nanotubes, which has higher-specific surface area,larger-surface coverage, excellent absorption and adsorption capabilityinto the porous gold-decorated fMWCNTs surface towards the target BPA.This significant change of surface current is monitored in everyinjection of the target BPA onto the Au-decorated fMWCNTs modifiedmicrochips by electrometer. I-V responses with Au-decorated fMWCNTsmodified microchip surface are investigated from the variousconcentrations (1.0 nM to 1.0 M) of BPA, which is showed in FIG. 5C. Itshows the current changes of fabricated Au-decorated fMWCNTs/microchipsas a function of BPA concentration in room condition.

It was also found that at low to high concentration of target BPAanalyte, the current responses were enhanced regularly. The potentialcurrent changes at lower to higher potential range (potential, +0.1 V to+1.5 V) based on various BPA analyte concentration are observed, whichis clearly presented in FIG. 5C. A large range of analyte concentrationis measured the probable analytical limit, which is calculated in 1.0 nMto 1.0 M. The calibration (at +0.5V) and magnified-calibration curvesare plotted from the various BPA concentrations, which are presented inthe FIG. 6. The sensitivity is estimated from the calibration curve,which is close to 5.7402 μA·cm⁻²·mM⁻¹. The linear dynamic range of thisAu-decorated fMWCNTs/microchips sensor displays from 0.1 nM to 10.0 mM(linearity, r²=0.9927) and the detection limit was considered as0.91±0.02 nM [3×noise (N)/slope(S)].

Example 12

BPA sensing mechanism on microchip sensor is executed due to thepresence of gold decorated functional MWCNTs in phosphate buffersystems. The resistance value of Au-decorated fMWCNTs/microchip isdecreased (current increased) with increasing surrounding active oxygen,which are the fundamental characteristics of materials. Actually,dissolved oxygen adsorption demonstrates a significant responsibility inthe electrical properties of the Au-decorated fMWCNT coated byconducting binders onto tiny microchip. The mechanism for the oxidationof BPA has been evaluated and presented in FIG. 7B. For BPA, themechanism was already reported in the literature. The mechanism processof BPA is controlled by the oxidation of BPA on the fabricated ormodified electrode surfaces. The initial stage in the oxidation ofphenols after 4-electron transfer leads to the formation of quinones.The electro-deposited products of BPA are proposed here to containseveral oxidizing centers which are known to contain selective o-quinoneor p-quinone via a four electron and four-proton process. The subsequentresponses which develop after the first evaluation were ascribed to theI-V responses of the deposited Au-decorated fMWCNT/microchips asobserved. Reaction mechanisms of BPA were proposed to contain severaloxidizing centers which are known to contain o-quinone or p-quinone viaa four electron and four-proton process. These proposed reactions areheld in bulk-system interface or adjacent conducting binder coated chipsdue to the small carrier concentration which enhanced the resistances.The BPA sensitivity could be attributed to the electrocatalysis occurredonto Au-decorated fMWCNTs/microchip and higher density conducts toincrease electron transfer. The larger quantity of electron transfer onthe fabricated sensor surface produces a larger oxidizing potential anda faster rate of oxidation of BPA. In the reaction system, thesereactions referred to oxidation of the BPA carriers in presence ofAu-decorated fMWCNTs/microchip systems. Consequently, resistance isreduced, and hence the conductance is increased. This is the cause whythe analyte response (current) amplifies with increasing potential from0 to +1.5V. Thus produced electrons contribute to rapid increase inconductance of the thin Au-decorated fMWCNTs/microchip film.

Example 13

The Au-decorated fMWCNTs/microchip unusual regions dispersed on thesurface would progress the capability of Au-decorated fMWCNT to absorbmore BPA species giving high resistance in ambient air. The sensorresponse time was ˜10.0 sec for the Au-decorated fMWCNT fabricatedmicrochip sensor to achieve saturated steady state current in I-V plots.The major sensitivity of Au-fMWCNTs/microchips sensor can be attributedto the good absorption (porous surfaces CNTs fabricated with binders),adsorption ability, and high-catalytic activity of the Au-decoratedfMWCNTs/microchips. The expected sensitivity of the Au-decoratedfMWCNTs/microchips fabricated sensor is relatively better thanpreviously reported BPA sensors based on other composites or materialsmodified electrodes. Due to large and active surface area, here theAu-decorated fMWCNT proposed a beneficial nanoenvironment for the toxicchemical detection (by adsorption) and recognition with excellentquantity using tiny microchips. The prominent sensitivity ofAu-decorated fMWCNTs/microchips affords high electron communicationfeatures which improved the direct electron communication between theactive sites of Au-decorated fMWCNT composed conducting binder coatedmicrochips. The modified thin Au-fMWCNTs/microchips sensor film had abetter reliability as well as stability in ambient conditions. Audecorated fMWCNTs/microchips exhibits several approaching in providingBPA chemical based sensors, and encouraging improvement has beenaccomplished in the research section.

Example 14

It was also investigated the sensing selectivity performances(interferences) with other chemicals like methanol, 3-methoxy phenol,BPA, ethanol, bromide, 4-aminophenol, and 1,2 dichlorobenzene etc. (FIG.8A). BPA exhibited the maximum current response by I-V system usingAu-decorated fMWCNT fabricated microchip electrodes. BPA exhibited themaximum current response by I-V system using Au-decoratedfMWCNTs/microchips compared to others chemical, which is presented inFIG. 8B. By deducting the current value of blank solution (at +0.5V), itis found the current value is less than 5% for all chemicals(3-methoxyphenol 1.5%, ethanol 2.1%, 4-aminophenol 4.0%, methanol 4.7%,1,2 dichlorobenzene 4.8%, bromide 3.8%, and blank (PBS only, 0%)),compared to target BPA (90.1%). It is specific towards BPA chemicalcompared to all other chemicals towards Au-decorated fMWCNTs/microchipssensor in phosphate buffer system.

Example 15

To investigate the reproducibly and storage stabilities, I-V responsefor Au-fMWCNTs/microchips sensor was examined (up to 2 weeks). Aftereach experiment, the fabricated Au-fMWCNTs/microchips substrate waswashed gently and observed that the current response was notsignificantly decreased (FIG. 8C). A series of five successivemeasurements of 0.1 μM BPA in solution yielded a good reproduciblesignal at Au-fMWCNTs/microchips sensor in different conditions with arelative standard deviation (RSD) of 1.8% (FIG. 8C).

The sensitivity was retained almost same of initial sensitivity up toweek (1^(st) to 2^(nd) week), after that the response of the fabricatedAu-fMWCNTs/microchips gradually decreased. The sensor-to-sensor andrun-to-run repeatability for 0.1 μM BPA detection were found to be 1.2%using Au-fMWCNTs/microchips. To investigate the long-term storagestabilities, the response for the Au-fMWCNTs/microchips sensor wasdetermined with the respect to the storing time. The long-term storingstability of the Au-fMWCNTs/microchips sensor was investigatedsignificantly at room conditions. The sensitivity retained 95% ofinitial sensitivity for several days. The above results clearlysuggested that the fabricated sensor can be used for several weekswithout any significant loss in sensitivity. The dynamic response (1.0nM to 10.0 mM) of the sensor was investigated from the practicalconcentration variation curve. The sensor response time is mentioned andinvestigated using this sensor system at room conditions. In Table 1, itis compared the performances for BPA chemical detection based on variousmodifications with different materials. It exhibits the highersensitivity using Au-decorated fMWCNTs/microchips compared othermaterials fabricated sensors with the similar target analytes.

TABLE 1 Linear Dynamic Sensitivity Limit of Technique/ Range (μA μMDetection Electrode fabrication Methods (LDR) cm⁻²) (LOD) pHChitosan-rapheme/ Voltammetry 8.0 nM to 1.0M —  6.0 nM — ABPE  SWNTs/β-cyclodextrin-GCE Amperometry 1.0 nM-1.85 μM —  1.0 nM —MWCNT/melamine/GCE CV 10.0 nM to 40.8 —  5.0 nM — μM Thionine/CPEAmperometry 0.15 μM-45.0 μM —  0.15 μM — Silica MCM-41/CPE DPV 0.22μM-8.8 μM — 0.038 μM — Au/Agcore-shell NPs Raman 00 nM to 10 fM —  10.0fM — Spetroscopy Magnetic NPs/CPE CV 6.0 μM-100 μM —  0.1 μM —Poly(2-aminothio- CV 0.6 μM-55.0 μM —  0.2 μM — phenol)/GCEβ-Cyclodextrin/CPE DPV 0.1 μM-11.0 μM —  0.83 μM — 1,10-PhenanNTf₂ ionicI-V 0.1 nM-0.1 mM 1.485 0.09-0.01 nM 7.1 liquid/AgE Au-fMWCNT/microchipsI-V 1.0 nM-10.0 mM ~5.7402  0.91 nM 7.1

Example 16—Advantages of the Chemical Sensor

This methods and detectors described herein provide an economic route tosimple, reliable, and cost effective electrochemical BPA chemicalsensors having AuNPs-decorated fMWCNTs/microchips by I-V method at roomconditions. It is accentuated that the above described embodiments ofthe present disclosure, described with the help of examples, aregenerally describing the disclosure. Many modifications and variationsmay be made to the above described embodiment of the invention withoutdeviating from the fundamental nature and scope of the invention.

This chemical sensor based on AuNPs-decorated fMWCNTs/microchips by I-Vmethod can detect BPA chemicals with I-V approaches. It is cheap, easyto handle, simple to prepare, and more effective sensor withAuNPs-decorated fMWCNTs/microchips.

1-14. (canceled)
 15. A method of determining an aqueous system BPAconcentration in a BPA-containing solution, comprising: contacting theBPA-containing solution with a working electrode and a counter electrodeof an electrochemical cell assembly; applying a voltage to the workingelectrode and the counter electrode to oxidize at least a portion of BPAin the BPA-containing solution to produce an electric current within theelectrochemical cell; and determining the aqueous system BPAconcentration in the BPA-containing solution based on the electriccurrent, wherein the electrochemical cell assembly, comprises: anelectrochemical cell comprising a working electrode, comprising agold-titanium alloy with a gold content of 70-90 wt %, based on a totalweight of the working electrode; gold-coated carbon nanotubescomprising: carboxylic acid functionalized carbon nanotubes, and goldnanoparticles bound to the carboxylic acid functionalized carbonnanotubes, wherein a diameter of each of the gold-coated carbonnanotubes is within a range of 5-20 nm, and wherein the gold-coatedcarbon nanotubes are in the form of a buckypaper with a thickness in therange of 100 to 500 μm; a conductive epoxy binder in the form of a curedlayer with a thickness of 1-3 mm, which is sandwiched between thegold-coated carbon nanotubes and the working electrode, that binds thegold-coated carbon nanotubes to the working electrode; a microchip stripon which the electrochemical cell is disposed; and a counter electrodedisposed adjacent to the working electrode having a gap therebetween.16. The method of claim 15, wherein the aqueous system BPA concentrationin the BPA-containing solution is within the range of 1.0 nM to 1.0 M.17. The method of claim 15, wherein the aqueous system BPA concentrationin the BPA-containing solution is determined in a time range of 5-20seconds after the contacting.
 18. The method of claim 15, wherein theBPA-containing solution comprises BPA and one or more of C₁-C₅ alcohols,C₁-C₅ alkoxy phenols, amino phenols, aryl halides, and halide ions, andthe method has a BPA selectivity of at least 90%.
 19. The method ofclaim 15, wherein the voltage is up to 2.0 V.
 20. (canceled)